Numerous problems are regularly encountered with use of conventional hot-melt glue. The temperature to which such glue must be heated (350.degree. to 400.degree. F.; 177.degree. to 204.degree. C.) virtually precludes battery (dry cell) operation. The high temperature corresponds to higher power consumption costs and longer preparation time; it also restricts the material from which the glue pot or other container can be constructed and thus increases cost in that respect as well.
All adhesives wet in two primary ways:
1) mechanical adhesion relates to microscopic surface roughness and penetration of adhesive into pores and irregularities on or wetting of a surface to which the adhesive is applied, and PA1 2) chemical adhesion relates to polarity associated with adhesive and substrate, as well as free electron attraction on the adhesive and substrate surfaces. PA1 a) Obtaining a hot-melt glue composition which is readily applicable at a temperature significantly lower than that of conventional hot-melt glue compositions without materially sacrificing bond-strength properties. PA1 b) Obtaining a hot-melt glue composition which is less expensive to prepare and/or process. PA1 c) Obtaining a hot-melt glue composition which does not soften at ambient temperature. PA1 d) Obtaining a hot-melt glue composition which can be applied at a temperature which does not burn skin. PA1 e) Developing a hot-melt formulation for a glue-gun glue stick with a heat resistance of a minimum of 125.degree. F. (51.5.degree. C.), and higher if possible, and with an application temperature (from a glue dispensing gun) of at most 280.degree. F. (138.degree. C.), preferably 250.degree. F. (121.degree. C.), or lower. PA1 f) Developing a hot-melt formulation suitable for a battery-operated glue gun. PA1 a) a hot-melt adhesive composition having an application temperature in an approximate range of from 180.degree. to 250.degree. F. and based upon an adhesive polymer preferably having a melt index in excess of 750 g/10 min.; PA1 b) a shaped adhesive glue stick suitable for use in a glue gun and consisting essentially of composition (a); PA1 c) a method of reducing the application temperature of a glue gun glue stick by using adhesive polymer which preferably has a melt index in excess of 750 g/10 min.; and PA1 d) a method of using adhesive polymer with a melt index in excess of 750 g/10 min. in compounding a hot-melt adhesive composition.
Mechanical adhesion is generally from 2 to 10 times stronger than chemical adhesion for those substrates which have higher surface roughness, such as wood and paper. Even on smooth surfaces, such as those of hard plastic, mechanical adhesion is usually at least equal to chemical adhesion. Mechanical adhesion depends upon viscosity and surface tension of molten hot melt. As high viscosity and high surface tension are antagonistic to wetting, hot melts have been applied at as high a temperature as possible without degradation in order to reduce both of these properties.
One particular use of hot-melt adhesive has been in the form of glue sticks for glue guns. When conventional hot-melt glue is used in such glue sticks, the glue must be heated to a temperature which often burns the fingers of the glue-gun operator. The temperature, which necessitates higher-cost glue-gun components, is also sufficient to melt or otherwise impair materials to which the hot-melt glue is applied.
These problems could be overcome by a glue stick which has a lower application temperature. In this regard, investigations have been made for glue compositions having lower application temperatures. Unfortunately, such compositions have been found lacking in bonding strength. To improve bonding properties, attempts have been made to increase the viscosity of such low-melting compositions. This has proved ineffective.
In order to accommodate hot-melt glue sticks, conventional glue guns must have a metal barrel through which the glue stick is conducted. The cost of a glue gun could be materially reduced if it could be made completely from plastic materials. The latter would have a further advantage of reducing electrical conductivity.
There are many known hot-melt adhesives with melting points below 180.degree. F. (82.degree. C.) These were designed with various components, which made them, e.g., pressure-sensitive, like that used in making "Scotch" tape. Such adhesives are very soft and often exhibit cold flow. In general, they are designed for automated application from large industrial bulk melt systems. Others are designed for glue pot applications.
The glue gun was originally designed in the 1960's. Even today, it has nominal operating temperatures in the range of from about 177.degree. to about 204.degree. C. All hot-melt glue sticks for glue guns are designed for these conditions, and currently-available glue guns do not operate at temperatures below this range.
Traditional hot-melt adhesive (used as a shaped adhesive for glue guns) is composed of a mixture of adhesive polymer, tackifying resin and wax. The adhesive polymer generally has a melt index of from 1.5 to 550 g/10 min. (dg/min.) and a melting point of from 180.degree. to 330.degree. F. (82.degree. to 166.degree. C.) The adhesive polymer is optionally composed of any of a number of base materials, such as polyester, polyamide, polyethylene, and polypropylene. Although characteristics of each base material are different, their application temperatures and viscosities substantially parallel each other.
The most widely used polymer in hot melt formulations is ethylene/vinyl acetate (EVA), which (when combined with resin and wax) provides generally good bonds to a wide variety of substrates. However, such EVA compositions are viscous in nature, and the normal recommended application temperature range of from 300.degree. to 450.degree. F. (149.degree. to 232.degree. C.), preferably 375.degree. F. (191.degree. C.), is consequently much higher than the softening point of the formulations. The disparity between softening point and application temperature is one of the primary problems addressed by the subject invention.
Traditional hot-melt adhesives are based on medium and/or low melt-index polymer adhesive, which has a higher molecular weight and a higher viscosity than its high-melt-index polymer adhesive counterparts. The higher viscosity and higher molecular weight have been regarded as crucial even to achieve hot-melt compositions yielding a bond strength barely above 200 psi in adhesive tensile and 140.degree. F. (60.degree. C.) in heat resistance (with a dead hanging 2-pound weight).
When traditional hot-melt adhesive is modified with a low-melting point additive to formulate a reasonably useful low application temperature hot-melt, certain characteristics are necessarily sacrificed. To compound a hot-melt composition with a significantly lower application temperature without materially adversely altering set time and strength has been previously sought after in vain.
Generally marketed hot-melt glue EVA compositions have a heat resistance of approximately 145.degree. F. (63.degree. C.) and a somewhat higher ring and ball softening point with a viscosity at application temperature (350.degree. F.; 177.degree. C.) of anywhere from 1,000 cp to 50,000 cp. Loading a composition with inert filler or using lower melt index EVA (with a much higher viscosity) to achieve higher heat resistance has resulted in raising the application temperature as well as the pressure required to extrude the resulting formulation from an applicator; such formulations result in a heat resistance gain of only about 10.degree. or 15.degree. F. (5.5.degree. or 8.5.degree. C.), but the application temperature must be raised approximately another 50.degree. F. (28.degree. C.)
There are many applications for bonding that do not require a heat resistance of 145.degree. F. (63.degree. C.) Many substrates, such as polystyrene foam and polyethylene film, cannot resist an application temperature of from 350.degree. to 450.degree. F. (177.degree. to 232.degree. C.)
Although the title of this application is COOL MELT GLUE, the compositions and their applications are still recognized as those of hot-melt adhesives. References herein to hot-melt glue thus do not necessarily distinguish over features of this invention.
In considering differences in properties, e.g. bond strength, between typical conventional hot-melt based glue sticks for glue guns and cool-melt based glue sticks of the subject invention, the particular substrate involved is a significant factor. Typical adhesive tensile strengths (adapted from ASTM D-1002 modified) [bond strength (in psi) data] for conventional hot-melt based glue sticks and cool-melt glue sticks are provided for a variety of substrates:
______________________________________ Subtrate Hot Melt Cool Melt ______________________________________ G-10 Phenolic 190 270-340 ABS 60 80-120 Nylon 35 140-200 Polycarbonate 60 130-180 Polypropylene 60 120-170 Styrene 30 50-80 ______________________________________
As readily appreciated from this table, the respective bond strengths achieved with the cool-melt glue are in excess of those obtained with a typical hot-melt glue by at least 30 percent. Bond strengths of that order of magnitude above those obtained with typical hot-melt glue sticks are regarded as excellent bond strengths.
In the preceding table the typical values for traditional hot-melt adhesives are obtained by applying the adhesive at 350.degree. F. (177.degree. C.), whereas the values reflected for the cool-melt glue are obtained by applying that glue at 200.degree. F.(93.degree. C.) There are traditional hot-melt adhesives which have competitive bond strength values, and some high performance cool-melt formulas which exceed the recited bond strength ranges. These are generally considered to be specialty hot melts intended for high bond strength applications and are exceptional.
Traditional hot-melt adhesives are frequently processed in melt kettles or reactors at temperatures around 350.degree. F. (177.degree. C.) Such temperatures are required by the melt index and the melting point of materials in the hot-melt adhesive blend. At those temperatures certain volatiles in employed tackifier resins gas off and begin to degrade. Over an extended period of time, e.g. about 12 hours, the resulting degradation produces a darkening in color. Char, also due to the degradation, occurs on the inside of the reactors.
In view of the temperatures involved, serious burns result when the molten material is handled carelessly. The high process temperatures also require use of a hot oil system to heat the reactors to 300.degree. to 350.degree. F.; the oil has to be heated to about 400.degree. to 450.degree. F. (204.degree. to 232.degree. C.) This type of process thus requires a sophisticated heating system and large amounts of energy; processing 8000 pounds per day requires 70,000 BTU/hr. Moreover, the work environment includes constant heat with a temperature of about 135.degree. F. (57.degree. C.) near the work area at the top of the reactors.
In contrast, a normal processing temperature for cool-melt glue is in the range of from 200.degree. to 230.degree. F. (93.degree. to 110.degree. C.) This fact alone provides a number of advantages. The kettle or reactor temperatures are low enough to eliminate almost all char and are also below the vaporization temperature of resin volatiles. The operating temperature also reduces or eliminates any short term degradation and hot-melt darkening. Handling problems are far less serious and virtually eliminate any serious burns. Environmentally, problems relating to produced vapors are eliminated. Also, it is likely that hot-oil heating can be replaced by steam or electric heating; this will reduce costs and safety risks inherent in a hot-oil heating system.
Traditional hot-melt adhesives are normally applied at a temperature between 350.degree. and 400.degree. F. (177.degree. and 204.degree. C.) This application temperature is needed to provide a low enough melt viscosity to ensure good wetting of the substrates and adequate open (work) time to position the several elements involved after glue is applied to them. Even pressure-sensitive hot-melts (hot-melts which have continuous tack even at room temperature) often have to be applied at 350.degree. F. (177.degree. C.) because their viscosity is too high to be applied effectively at lower temperatures.
Specific problems occur at the noted application temperature range. Traditional hot-melts char and darken within the noted range in the same manner as during processing. The char can clog nozzles, and bond strengths decline as tackifiers degrade. Some tackifiers contain residual volatiles which will out-gas and cause bubbles of gas in the hot melt. This interferes with nozzle shutoff, as bubble pressure lifts (opens) the nozzle valve and results in a leaking nozzle. Since many of the produced gases are acidic, they can attack some applicator (glue gun) components, such as the silicone sleeve and seals, reducing their durability. The same safety issues remain; as a worker must apply glue to and handle glued items, there is a continual risk of incidental or consequential burns.
When hot-melt or cool-melt glue sticks are prepared for glue guns, the application temperature is a material factor. The outside handle set of the glue gun must be designed to limit the outside surface temperature. The location of the casting must be far enough away from the hand to prevent the handle area from being "too hot to handle". U.L. certification also requires that material used in the handle set be adequate to withstand the temperatures involved at the casting area (the highest temperature region). Such material tends to be more expensive and harder to mold than less sophisticated materials certified for use in lower temperature applications. Some of the materials, such as the silicone used in the sleeve, have reduced ratings at the in-use temperature of 400.degree. F. and thus reduced durability. Alternative choices are limited.
A temperature gradient from 400.degree. to 75.degree. F. (204.degree. to 24.degree. C.) exists. This temperature transition occurs between the casting and the end of the silicone sleeve in a glue gun. Over the involved distance glue goes from molten to solid. The required length over which this change occurs is from about one inch to about 1.5 inches, depending upon the glue formula and the casting temperature. Under long-term steady-state operation there is molten glue within the sleeve area. When the glue gun is unplugged and cooled, molten glue in the sleeve hardens. Upon reheating, the glue does not feed through the sleeve until thus hardened glue is resoftened. Heat must migrate from the casting into the sleeve area, which requires time and extends the period before which the glue gun is capable of feeding a glue stick through the sleeve. The shorter the transition distance, the quicker the glue gun is ready to use.
With conventional hot-melt glue stick compositions additional problems arise with regard to glue-stick feeding in a glue gun. Even though a silicone sleeve provides means for the glue to go from liquid to solid, convective heat from the casting (the glue stick) flows toward the rear of the sleeve, and air at the entrance to the sleeve can be at a temperature of from 100.degree. to 120.degree. F. (38.degree. to 49.degree. C.) This temperature is often sufficient to soften glue-stick formulations, which thus become subject to jamming in the feeding mechanism or, in the extreme, actually become semi-liquid. When a glue-stick durometer reading drops to less than a Shore "A" hardness of 35, such feeding problems regularly occur with conventional feed mechanisms. The correlation between hardness and glue-stick temperature is reasonably linear.
The cool-melt glue formulations of this invention eliminate most of the previously-noted problems. Their application temperature is normally within the range of from 180.degree. to 230.degree. F., (82.degree. to 110.degree. C.), which reduces or eliminates the generation of char and at least retards any possible degradation. Most of the tackifiers used in cool-melt glue formulations are the same as those used in traditional hot-melt glue formulations. Their degradation is directly related to the production and application temperatures, as with almost all oxidation reactions. The cooler casting temperatures for the cool-melt formulations also eliminate the out-gassing problems, as volatiles in the tackifying resins usually do not vaporize below 300.degree. F. (149.degree. C.) With a casting temperature of 200.degree. F. (93.degree. C.) available certified materials for use in glue guns are more numerous and less expensive. There is greater latitude in glue gun design because the user does not have to be protected from high temperatures. A glue gun can be designed with its handle grip area much closer to the casting than is possible with conventional glue guns. The hand grip area of a glue gun can even surround the casting; it can resemble a syringe having a glue stick as its plunger. At 200.degree. F. (93.degree. C.) cool-melt formulations have few dangerous handling problems; in fact, only slight discomfort would be encountered when such cool-melt is applied directly to an unprotected hand. At a casting temperature of 200.degree. F. (93.degree. C.) materials, such as silicone, are well below their rated temperature limits and are extremely durable.
The temperature gradient with cool-melt glue stick formulations is from 230.degree. to 75.degree. F. (110.degree. to 24.degree. C.) While there is still a transition from molten glue to solid glue, the temperature range and the required distance are significantly reduced. The distance needed is only from about 0.1 to 0.4 of an inch, even under steady-state heated conditions. As a result there is almost no delay in the start-up of a glue gun because the glue stick can feed as soon as the casting is at temperature; heat does not have to migrate far into the sleeve. Additionally, convective heat from the glue-stick casting has almost no impact upon the temperature of the air at the entrance of the silicone sleeve. Feeding problems are thus reduced, as the glue sticks are not exposed to air temperatures, which could lower their durometer readings significantly below values at ambient temperature.